1. Field of the Invention
This invention relates to a mold assembly for resin sealing which is employed in the manufacture of a semiconductor device, etc.
2. Description of the Prior Art
In general, in the resin sealing process for transistors, ICs etc., resin flows out of a resin mold portion, and an abnormal protuberance (fin) is created on the surface of a lead outside the mold portion. The protuberance is an obstacle to the operation of plating, soldering, etc., of the lead, to be executed later, and is unfavorable from the viewpoint of the external appearance of a product.
A conventional mold assembly for resin sealing will be described with reference to FIG. 1, which shows an upper mold 1 and a lower mold 2. The lower mold 2 is composed of a holder 3 and cavity blocks 4. The fin referred to above develops when there exists a clearance 6 of the parting surface between the upper mold 1 or lower mold 2 and a respective lead frame 5b or 5a. That is, the fins 7a are usually formed when parts of a resin 7 injected in a space which is defined by the upper mold 1, the lower mold 2 and the two lead frames 5a and 5b percolate through the clearances 6. Such fins are formed, not only at the right and left in FIG. 1, but also in front and in the rear. In order to prevent the formation of such fins 7a, accordingly, the clearance 6 must be removed over the entire periphery of the parting surface.
As one technique for eliminating the clearance 6 and preventing the formation of the fin 7a, there has hitherto been adopted a method wherein the parting surface is endowed with a high degree of flatness by finishing it with a high degree of high precision, for example, with 5.mu. of flatness deviation, a method wherein the upper mold 1 and the lower mold 2 are designed so as to become high in rigidity and only slightly in deformable, or a method wherein a heat treatment for eliminating working distortion, hardening distortion, non-uniformity of the hardened structure, etc., of the material of the upper mold 1 and the lower mold 2 is accurately executed with the greatest possible care.
Even with these prior-art methods, however, the clearance appears at the parting surface between the upper mold 1 or lower mold 2 and a respective lead frame 5a or 5b as before and the development of the fin cannot be perfectly prevented where the respective parting surfaces of the cavity block 4 and the holder 3 are not flush with each other and have a step therebetween, where a plurality of cavity blocks 4 are employed and where the parting surfaces of the respective cavity blocks 4 have errors in flatness, where, when a plurality of lead frames are sealed at the same time, a difference in thickness exists between, for example, the lead frames 5a and 5b.
On the other hand, technical limitations limit the working dimensional precision of the upper mold 1 and lower mold 2, and a high cost is required to make the thicknesses of all the lead frames uniform. It has therefore been the situation that the appearance of the fin is unavoidable.
As an expedient for solving such a problem, applicants have proposed a mold assembly for resin sealing including a pair of upper and lower molds, characterized in that an elastic deformation portion adapted to be deformed along a compressive direction is provided at least either between an upper surface of a cavity block of the upper mold and an upper mold holder or between a lower surface of a cavity of the lower mold and a lower mold holder, the elastic deformation portion being deformed at the resin sealing, whereby parting surfaces of the cavity blocks and holders of said upper and lower molds and surfaces of a lead frame can be brought into close contact (Japanese Patent Application Public-Disclosure No. 11772/1975).
The proposed mold assembly for resin sealing and a method of molding with the mold assembly will now be briefly explained with reference to FIG. 2 and FIGS. 3a and 3b.
FIG. 2 is a perspective view showing a part of a cavity block 8 for use in the resin sealing mold assembly proposed, the cavity block being illustrated in the reversed state. In the figure, numeral 9 designates a projection for deformation (elastic deformation part), and a plurality of projections 9 are formed on the bottom of the cavity block 8 in a balanced arrangement of the positions thereof. The projection for deformation 9 can be elastically deformed in the vertical direction by a mold clamping pressure at the step of resin sealing. Stresses which are exerted on the deformable projections 9 at the resin sealing become repetitive stresses. The number, size, etc., of the deformable projections 9 must accordingly be selected so that the stresses may have a stress value within the compression fatigue limit of the material (for example, alloy tool steel) of the cavity block 8. Shown at 10 is a surface for preventing deformation, which is formed on the bottom of the cavity block 8 similar to the deformable projections 9 in a manner to be somewhat (indicated by l in FIG. 3a) lower than the level of the surfaces of the projections 9. The surface 10 serves to limit the amount of deformation of the deformable projections 9 so as to prevent damage thereto. The parting surface of the cavity block 8 is made higher than that of the holder 3 by a value x indicated in FIG. 3a beforehand in order that clearances caused by errors in flatness of the parting surface, etc., may be absorbed by the deformable projections 9 at the mold clamping. The value l is made greater than the value x.
FIGS. 3a and 3b illustrate a method in which lead frames are resin-sealed by the use of the resin sealing mold assembly having such cavity blocks 8. FIG. 3a shows a sectional structure of the state in which the lead frames 5a and 5b are placed on the lower mold 2 with the cavity blocks 8 integrally assembled in the holder 3 of the lower mold 2, the lead frames being subsequently subjected to the mold clamping by the upper mold and the lower mold. As is shown in FIG. 3b, the lead frames 5a and 5b are held between the upper mold 1 and the lower mold 2, and the load of a mold clamping pressure P is exerted on the upper mold 1 and the lower mold 2. Before exerting the load of the mold clamping pressure P (in the state of FIG. 3a), the parting surfaces of the cavity blocks 8 lie at a level higher by the value x than the parting surface of the holder 3, and hence, the mold clamping pressure P is applied to the parting surfaces of the cavity blocks 8. Thus, the deformable projections 9 provided on the bottoms of the cavity blocks 8 are elastically compressed and deformed by the clearance x, and the respective parting surfaces of the cavity blocks 8 and the holder 3 come into agreement. Accordingly, when the resin 7 is injected under this state, the fins of the resin as in the prior art are not formed. Even when the lead frames 5a and 5b have a difference in thickness, the deformable projections 9 undergo elastic deformation so as to eliminate the clearances between the lead frames 5a, 5b and the parting surfaces of the cavity blocks 8, and hence, the generation of the fins can be prevented similarly to the above.
However, this technique is not problem-free. Even with such a resin sealing mold assembly, where the lead frame is locally thick or thin in a range of several .mu.m -- several tens of .mu.m within the single lead frame, especially where the surface of the lead frame swells, it is extremely difficult to bring the parting surfaces of all the cavity blocks and the holders of the upper and lower molds and the surfaces of the lead frames into close contact according to the swell, with the result that thin resin fins develop.
Further, as shown in FIG. 4a, the prior-art mold assembly for resin sealing has a step indicated at A between that parting surface 11 of the holder 3 for receiving the cavity block 8 which comes into contact with the upper mold 1 and the surface 12 with which the lead frame is to be closely contacted when the deformable projections 9 of the cavity block are compressed by x by the mold clamping. Accordingly, where the lead frame 5c, whose thickness t is smaller than the value of the step A, is sealed by such a mold assembly, the cavity block 8 is deformed only by the value of the height t.sub.1 by which the lead frame projects beyond the parting surface 11. As is shown in FIG. 4b, therefore, even when the upper mold and the lower mold are clamped by the mold clamping pressure P, a clearance B, having a valve of (A - t), arises between the lead frame and the surface 12, and the resin fin develops at the clearance part.